1. Field of the Invention
The present invention relates to a combined display panel for displaying various information thereon, and more particularly to a combined display panel in which a transparent display panel is disposed in front of a part of a surface of an instrument panel for use in an automotive vehicle, for example.
2. Description of Related Art
Combined display panels in which a transparent display panel such as a liquid crystal or electroluminescent display panel is disposed on the front surface of a usual instrument panel have been proposed hitherto. For example, Japanese Utility Model Publications No. Sho-55-16746 and No. Hei-3-17437 disclose a combined display panel having a transparent display panel of a liquid crystal or an a electroluminescent panel disposed in front of a conventional instrument. Display devices of this kind are able to display images displayed on the transparent panel in addition to images displayed on the conventional panel.
Japanese Patent Laid-Open Publication No. Hei-7-271310 discloses a combined display device of the same kind. This device turns off the display on a back panel when a front panel display is activated, and vice versa. In other words, images on the back and the front panels are selectively displayed. However, in an instrument panel of an automotive vehicle which displays various information on the same panel, including information such as vehicle speeds which have to be displayed always and information such as engine rotational speeds which are not necessary to be displayed always, it is not possible to turn off all the images displayed on the back or front panel. It is possible to overlap the front panel only on a part of the surface of the back panel where information which is not required to be displayed always is displayed, and to activate selectively the back and front panels. For example, a transparent front panel may be overlapped only on the surface of the back panel where a tachometer (engine speed indicator) is located and the surface of the back panel where a speedometer is located is left open without overlapping the front panel, and both images on the back and front panels may be selectively switched on or off.
In this case, however, there is a problem that luminance or brightness of the display device between the overlapped portion and the non-overlapped portion becomes different, because the transparent panel is not perfectly transparent but its transparency is usually less than 80%. This is because the transparent panel such as a liquid crystal or an electroluminescent panel is composed of many layers which are not perfectly transparent. The imbalance of luminance is harmful to a quality display.